I think have the mom there, then she knows what exactly you've said to the daughter (in case the child misunderstands) and she now has more answers ready. I'd also suggest the mom bring her daughter to talk to the Rabbi too to get answers from her own religion. Also, be open to the child thinking of answers, don't just automatically give an answer. Ask the child if she has any thoughts or answers to her own question. You might also want to look up any books for children on answering questions about religion which pertain to Judaism. I know there are lots of books out there for Christian kids, so there must be some for Jewish kids. Also, be sure what type of answers the mom is looking for. Is she looking for you to affirm thieir religion, to affirm all religions or no religion or just explain what other religions believe? Maybe the mother doesn't really understand how complex the issue can be or maybe she does want her daughter to have a more worldly view.